This invention relates to a time interval measuring device for measuring the unknown time interval between optical pulses.
In a conventional long distance measuring technique, or typically in a surveying technique, optical interference is utilized for accurate measurement. That is, in the method, a laser beam modulated with an electrical signal is caused to go and return for a distance to be measured, and the laser beam returned is demodulated, so that the phase of the modulating signal is compared with that of the original one. The difference in phase is converted into a time interval, from which the distance is calculated.
Furthermore, a method is also known in the art that an optical pulse is allowed to go and return for a distance to be measured, and the distance is calculated from the period of time required for the optical pulse to go and return for the distance.
However, the conventional method in which the phase comparison is carried out is disadvantageous in that, in the case where the weather conditions of the light transmitting space change with time, the light beam returned changes in amplitude, which results in an error in phase, thus limiting the measurement.
In the latter method in which the time required for an optical pulse to go and return for a distance to be measured is detected to measure the distance, the measurement is substantially free from the change of the amplitude which is caused by the variations of the weather conditions. However, heretofore the method is not sufficiently utilized because there has not been provided means for accurately measuring the difference between the time required for an optical pulse to go for a distance to be measured and the time required for the optical pulse to return for the distance.
If a photo-diode is used as in the conventional method, the time interval between two optical pulses can be measured with an error of about 100 ps. If a streak camera is used, then the time interval can be read with an error of about 1 ps. However, the time required for an optical pulse to go and return for a long distance is longer than that, and for instance a period of time of the order of 100 .mu.s cannot be measured with that accuracy. The reason for this resides in that, in an electrical circuit for determining the time positions of electrical pulses obtained through photo-electric conversion with gate circuits therein and accumulating for the time required till an end of measurement, the error in time determination or the timing fluctuation of the gate circuit readily reaches about 100 ps, and these errors are accumulated with the gating operation.